Foamed products have been made on an industrial scale for many years, one such common product being "sponge" rubbers that are made from latices of natural or synthetic rubber. At the outset, these products were formed by beating air into the uncured material and thereafter curing the material while in a foamed state. This beating or whipping process is done in a relatively slow batch-type process rather than a continuous basis and it does not yield good control over cell size, as the bubbles of air whipped into the material by this process tend to be relatively large.
In order to carry out foaming on a continuous basis, other systems have been proposed. One of these is a chemical approach in which a material capable of liberating a gas under certain predetermined conditions, known commonly as a "blowing agent," is introduced into the uncured material. Such systems involve additional cost because of the necessity for the blowing agent; in some instances, the blowing agent or its catalyst leaves undesirable residual materials in the foam, and chemically blown foams tend to have nonuniform cellularization across the cross section of the foam that results from unevenly mixed blowing agent. This latter characteristic is especially evident when the foams are metered onto a substrate and allowed to expand to their free rise density.
Another approach that has been used with substantial success involves the use of a rotary mixer that employs a rotor traveling at high speed. A liquid and gas are introduced, either separately or together, into a housing within which the rotor spins. The liquid/gas mixture flows through a shearing zone that is formed between cooperating surfaces on the rotor and the housing. In the shearing zone, the gas is subdivided into small bubbles to form a froth or foam. Some mixer designs employ a single rotor surface cooperating with a single stator surface to effect shearing, while other designs employ a double-sided rotor that cooperates with two opposed stator surfaces. However, it has not always been possible to achieve desired low densities at high production rates and to produce products having a fine cell structure with such equipment. Also, it has been found that some materials cannot be successfully foamed by such equipment.
Attempts have been made to overcome the shortcomings of single-head mixers by connecting at least two single-headed machines together, with the outlet of the first mixer supplying the inlet of a second mixer. However, even with such arrangements, desired low densities, fine cell size, and the ability to process certain difficult materials have not been forthcoming.